wxr import: don't load the whole file at once

Description

The wxr importer currently loads all posts into a large array before processing them. That makes large imports impossible on servers with a small memory limit.

The enclosed patch refactors import/wordpress.php to keep only one post at a time in memory. I've keep most of the original code intact to minimize breakage. It's no faster or slower than before. There are other optimizations that could be made but this is a necessary first step.